1. Field of the Invention
The present invention relates to an image forming apparatus having a stepping motor arranged in a conveying path for a paper, and a method for controlling the stepping motor in the image forming apparatus.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a printer and a complex machine, a stepping motor has often been employed as means for conveying a paper. This is because there is an advantage that the stepping motor allows so-called open-loop positional control that does not require a rotation sensor and the like. Although the stepping motor allows this open-loop positional control, a phenomenon of step-out occurs when load torque exceeding maximum output torque is applied even for a moment, and the stepping motor stops. Therefore, the stepping motor must be controlled to prevent this step-out.
Therefore, in the stepping motor used for paper conveyance in the image forming apparatus, for example, peak load torque is assumed in advance in consideration of variations in apparatus, the environmental temperature and the durability of the apparatus. A method for preventing the step-out by setting the magnitude of a drive current supplied to the stepping motor such that the maximum output torque having a predetermined margin can be achieved with respect to the assumed peak load torque has been generally employed.
The following techniques have been known as the foregoing method for driving the stepping motor while preventing the step-out.
Japanese Laid-Open Patent Publication No. 2001-262938 discloses a configuration including a stepping motor to which a positional sensor is added, in which a motor drive current is made variable in accordance with an amount of deviation between a command pulse and an amount of motor movement.
Japanese Laid-Open Patent Publication No. 06-133593 discloses a configuration in which control is performed over the speed of a stepping motor based on a difference between a load current in the middle of the rising time and the falling time of an applied pulse and a load current immediately before the falling time.
Japanese Laid-Open Patent Publication No. 2000-341994 discloses a configuration in which a step transition state of a rotor of a stepping motor is detected using a change in inclination of an excitation current, and a preset chopping constant current value corresponding to a maximum load of an apparatus is reduced to supplied excitation current.
Japanese Laid-Open Patent Publication No. 2002-010689 discloses a configuration in which the rising time of a drive current waveform of a pulse motor is changed depending on necessary torque by monitoring a drive current for every step by a current detecting circuit and changing a drive voltage by a variable power supply such that a necessary and sufficient current flows constantly.
Japanese Laid-Open Patent Publication No. 2002-064996 discloses a configuration in which it is determined whether or not a stepping motor is rotating normally at present, whether or not there is a risk of step-out, and the like by detecting a change in drive current of the stepping motor, determining a load and producing a signal.
Japanese Laid-Open Patent Publication No. 2004-104888 discloses a configuration in which, when an amount of operation of a stepping motor is equal to or larger than a set value, a target current value provided to a driver is sequentially changed and a critical step-out target current value of the motor is searched for.
However, the above-described conventional method for controlling the stepping motor and the above patent documents have the following problems.
When the drive current of the magnitude corresponding to the peak load torque assumed in advance is set as in the above-described conventional control method, more torque than necessary is produced and the efficiency is decreased as compared with other conveying means such as a DC brushless motor. In addition, since the current supplied to the stepping motor increases in consideration of a certain level of margin, the temperature of the stepping motor and the driver (in particular, a switching element therein) driving the stepping motor rises greatly and the rated size must be increased or a cooling measure must be taken in order to enhance the heat resistance. Furthermore, vibration occurs due to excessive torque at the time of light load and the like, and it is difficult to handle load fluctuations when an unexpected paper is used, for example.
In addition, in the control method disclosed in Japanese Laid-Open Patent Publication No. 2001-262938, the positional sensor is required for the stepping motor, and thus, the original advantage that the open-loop positional control is possible is lost. In addition, a high-speed processor for constantly monitoring the deviation and controlling excitation is required, which may cause a problem of an increase in cost.
In addition, in the control method disclosed in Japanese Laid-Open Patent Publication No. 06-133593, the rotation speed of the stepping motor changes, and thus, this method is not suitable for a configuration in which a paper is conveyed at a predetermined conveyance speed like the image forming apparatus. In addition, a control computation must be performed in a cycle comparable to a pulse rate (approximately several kilohertz to tens of kilohertz), which may cause a problem of an increase in cost. Furthermore, it is difficult to apply this method to a stepping motor of an excitation type other than 1-2 phase excitation.
In addition, in the control method disclosed in Japanese Laid-Open Patent Publication No. 2000-341994, the control computation must be performed in a cycle sufficiently shorter than the pulse rate, which may cause a problem of an increase in cost due to the processor.
In addition, in the control method disclosed in Japanese Laid-Open Patent Publication No. 2002-010689, a power supply voltage itself must be controlled and a relatively expensive power supply device (regulator) or the like is required, which may cause a problem of an increase in cost.
In addition, in the control method disclosed in Japanese Laid-Open Patent Publication No. 2002-064996, the control computation must be performed in the cycle comparable to the pulse rate, which may cause a problem of an increase in cost.
In addition, in the control method disclosed in Japanese Laid-Open Patent Publication No. 2004-104888, instantaneous load fluctuations cannot be handled.